In FY 93 our study of the myosin crossbridge interaction with actin in muscle fibers focused on the differences between the weakly-binding and strongly-binding crossbridge interactions with actin. The hypothesis was formulated that the major difference between weakly- and strongly-binding crossbridges is that the heads of the former are mobile after attachment whereas those of the latter are not. This explained almost all of the known differences in behavior between weakly- and strongly-binding crossbridges, including why the kinetics of weakly-binding crossbridges are not independent of ionic strength while those of strongly-binding crossbridges are not. It also explained why weakly-binding crossbridges relax tension about as quickly as myosin subfragment-1 detaches from actin in solution, but strongly-binding crossbridges, under most conditions, do so much more slowly. The molecular structural properties of myosin important in determining whether it binds weakly or strongly were investigated by examining the reaction sites for N-phenylmaleimide, a compound which, when it reacts with myosin, locks it in the weakly- binding configuration. It was found that N-phenylmaleimide alkylates myosin heavy chain at Cys-697 and Cys-707.